Paleostress Directions Near Two Low-Angle Normal Faults: Testing Mechanical Models of Weak Faults and Off-Fault Damage GEOSPHERE; V

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GEOSPHERE Paleostress directions near two low-angle normal faults: Testing mechanical models of weak faults and off-fault damage GEOSPHERE; v. 11, no. 6 Gary J. Axen1, Amy Luther1,*, and Jane Selverstone2 1Department of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA doi:10.1130/GES01211.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

12 figures ABSTRACT for slip in the upper crust, in which normal-friction INTRODUCTION CORRESPONDENCE: [email protected] LANFs are embedded in stronger surroundings

Many large-slip faults, such as the San Andreas and slip at high angles to σ1 and models of stress Low-angle normal faults (LANFs) are consid- CITATION: Axen, G.J., Luther, A., and Selverstone, fault and low-angle normal faults (LANFs), appear rotation across the thickness of the brittle crust, ered to be weak because they apparently slip at low J., 2015, Paleostress directions near two low-angle normal faults: Testing mechanical models of weak to be weak relative to their surroundings or to lab- with moderately plunging σ1 near the brittle-plastic resolved shear traction and at a high angle to the

faults and off-fault damage: Geosphere, v. 11, no. 6, oratory friction measurements, and to be poorly transition, provided that some mechanism allows regional maximum principal stress (s1) direction p. 1996–2014, doi:10.1130/GES01211.1. oriented for slip in the regional stress field. Sev- the faults to propagate through the brittle crust at (Reynolds and Lister, 1987, 1990; Axen and Selver-

eral models seek to explain the mechanics of slip gentle dips as the footwalls are exhumed. Paleo-σ1 stone, 1994; Axen, 2004). At typical fault friction Received 12 June 2015 and/or formation of such faults. Other models ex- vectors oriented at moderate angles to the faults values (m = 0.6–0.85; Byerlee, 1978), slip should not Accepted 10 September 2015 Published online 2 October 2015 plain damage around faults as due to fault or earth- are sparse and may reflect early damage formed occur in an Andersonian stress field unless other

quake rupture propagation or slip on nonplanar in the midcrust, while the angle between σ1 and factors are important, yet much horizontal crustal faults. Most of these models explicitly predict the the detachment was moderate or during along- extension in the world is accommodated on low- near-fault stress field. strike LANF or earthquake rupture propagation. angle detachments (Wernicke, 1981, 1992, 1995; Exhumed footwalls of low-angle normal faults Coulomb plasticity due to granular flow, which Wernicke et al., 1985; Howard and John, 1987;

are advantageous natural laboratories for test- predicts faults at ~45° to σ1, is not well supported Davis, 1988; Davis and Lister, 1988; Spencer and

ing such models because they expose rocks that because many paleo-σ1 vectors with moderate Reynolds, 1989; Axen et al., 1990, 1993; Axen, 1993; passed through the brittle-plastic transition and angles to the LANFs are from fractures below the Abers et al., 1997; Axen and Fletcher, 1998; Axen, all or part of the seismogenic crust. We present cataclastic fault cores. Our results are inconsistent 2004; Collettini et al., 2006; Reston, 2009; many reduced paleostress tensors derived from inver- with “weak-sandwich” models that predict re- others). The San Andreas fault also appears to be a

sion of fracture and slip-line orientation data taken orientation of σ1 to low angles (~30°) to the fault weak fault that slips at a high angle to the inferred mainly from the fault cores and fractured damage within the damage zone and/or fault core due to maximum horizontal stress (Lachenbruch and Sass, zones in the upper footwalls of two LANFs, the local pore-fluid pressure or elasticity changes. Frac- 1980; Mount and Suppe, 1987; Zoback et al., 1987; Whipple and West Salton detachment faults of turing due to slip on non-planar faults is generally Lachenbruch and McGarr, 1990; Rice, 1992; Harde southern California. Frictionally weak materials consistent with our paleostress results. However, beck and Hauksson, 2001) and may share similar probably were not significant along these faults the roughness of the LANFs studied is not known, mechanical properties to LANFs; therefore, this except in the uppermost few kilometers of the but they may have very low roughness. The stress study has broad implications for fault mechanics. crust, and pore-fluid pressure probably never ap- state used in this wavy-fault model to constrain Several testable hypotheses have been pro- proached lithostatic values. the expected damage region is nearly identical to posed to explain the weak-fault paradox; most hy- Most results show that the faults were at a that inferred in the strong-sandwich model from potheses were developed for strike-slip faults and high angle to the near-fault maximum compres- field measurements. Fractures in the damage zone others for low-angle normal faults. Many hypoth-

sive stress (σ1) direction, in general accord with probably do not record up-dip fault or earthquake eses are applicable to both fault types. (1) Low-fric- Andersonian extensional stress fields. Our results rupture propagation, which is expected espe- tion materials, such as aligned clay minerals, are support a “strong-sandwich” mechanical model cially for earthquake propagation, but along-strike common on fault surfaces or in fault cores (Numelin propagation may have controlled fracturing at et al., 2007; Collettini et al., 2009; Carpenter et al., some sites. Some paleostress fields are probably 2011; Haines and van der Pluijm, 2012; Lecomte *Present address: Department of Geology and Geophysics, For permission to copy, contact Copyright E235 Howe-Russell-Kniffen, Louisiana State University, Baton related to folding of the detachments about slip- et al., 2012). (2) High pore-fluid pressure, causing Permissions, GSA, or [email protected]. Rouge, Louisiana 70803, USA; [email protected]. parallel axes. low effective normal stress, can reduce apparent

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friction (Hubbert and Rubey, 1959). (3) Dynamic shear- and fault-zone rocks that are the products sets with focal mechanisms that are too similar for weakening processes may reduce friction during of fault-zone processes beginning near or below rigorous inversion. seismogenic slip (Brodsky and Kanamori, 2001; the crustal brittle-plastic transition and overprinted Wibberley and Shimamoto, 2005; Rempel, 2006; during passage upward through the seismogenic Rice, 2006). (4) Physical-mechanical differences brittle crust (e.g., Davis, 1983; Wernicke, 1985; WEAK-FAULT AND FAULT-DAMAGE between fault-zone rocks and their surroundings Reynolds and Lister, 1987; Davis, 1988; Axen and MODELS may allow slip on misoriented faults (Figs. 1–3; Selverstone, 1994; Selverstone et al., 1995; Waw- Mandl and Fernandez Luque, 1970; Mandl et al., rzyniec et al., 1999, 2001; Cowan et al., 2003; Col- Low-Friction Materials 1977; Axen, 1992; Byerlee and Savage, 1992; Rice, lettinni and Holdsworth, 2004; Hayman, 2006; Nu- 1992; Lockner and Byerlee, 1993; Axen and Selver- melin et al., 2007; Smith et al., 2008; Selverstone Low-friction materials are typically sheet sili- stone, 1994; Marone, 1995; Faulkner et al., 2006; et al., 2012). cates such as chlorite, illite, smectite, talc, or ser- Healy, 2008, 2009). Most of these models require We present inversions of macroscopic fracture pentine (e.g., Byerlee, 1978). Such materials have

rotation of s1 to a lower angle to the detachment and slip-vector orientations from the uppermost been found along several low-angle normal faults within the width of the fault zone (Mandl and Fer- parts of the footwalls of two low-angle normal (Numelin et al., 2007; Collettini et al., 2009; Carpen- nandez Luque, 1970; Mandl et al., 1977; Axen, 1992; faults (the Whipple detachment fault [WDF] and the ter et al., 2011; Lecomte et al., 2012; Haines and van Byerlee and Savage, 1992; Rice, 1992; Lockner and West Salton detachment fault [WSDF], southern der Pluijm, 2012), and the weakness is enhanced by Byerlee, 1993; Marone, 1995; Faulkner et al., 2006). California) to obtain local paleostress orientations. fault-core foliations subparallel to the slip surfaces (5) In contrast, stress rotation may occur across the Fractures were measured at sites mostly from (e.g., Collettini et al., 2009). For typical continen-

thickness of the brittle crust, with s1 in a subverti- within the faults’ damage zones or breccia-ultra tal LANFs such as the WDF or WSDF, alteration to cal Andersonian orientation in the shallow crust cataclastic cores, within meters to tens of meters clays (as opposed to serpentine or talc) is expected and rotated to moderate plunges at and below the of the faults, with fewer sites below those levels and this is thought to occur in the upper ~5 km brittle-plastic transition, due to underlying plastic providing “background” comparisons. These of the crust (e.g., Numelin et al., 2007), or at tem- shear (Fig. 4; Yin, 1989; Buck, 1990; Westaway, 1999, paleostress-field orientations can be compared to perature <180 °C (Haines and van der Pluijm, 2012; 2001) or crustal thickness variations (Spencer and predictions made by several of the models above, yielding, for example, a maximum depth of 6 km Chase, 1989). Such rotation would aid LANF slip in specifically types 3, 5, 6, and 7. The models are dis- for a geotherm of 30 °C/km). Thus, it appears that the deeper brittle crust, where slip is most difficult cussed in the Weak Fault and Fault Damage sec- weak, foliated gouge is unlikely to aid reduction (Axen, 2004), but does not explain LANF slip in the tion ; the geology of the Whipple and West Salton of LANF strength in the strong, seismogenic crust shallower crust (e.g., Wernicke et al., 1985; Axen, detachment faults is described in the Whipple and (~5–12 km depth). 1993). In addition, two other models suggest that West Salton Detachment Faults section; and meth- stresses may be reoriented near faults and may ods of inversion are discussed in the Paleostress In- control fault-zone damage: (6) near-field stress rota- version section. Paleostress results are compared Elevated Pore-Fluid Pressure tion produced by fault propagation (Fig. 5; Vermilye to the models in the Results section.

and Scholz, 1998) and/or earthquake rupture prop- Similar stress-field inversions have been High pore-fluid pressure (Pf) has long been agation (e.g., Rice et al., 2005) and (7) near-field done around the San Andreas fault system using understood to aid fault slip by reducing the effec- stress perturbations due to fault roughness (Fig. low-magnitude seismicity (e.g., Jones, 1988; Harde tive normal stress on faults (Hubbert and Rubey, 6; Chester and Chester, 2000). Most models of all beck and Hauksson, 1999; Townend and Zoback, 1959). This mechanism may explain the lack of a types above were developed from a combination of 2001; Provost and Houston, 2001, 2003; Hardebeck heat-flow anomaly across the San Andreas fault materials theory and experimental rock-mechanical and Michael, 2004). However, these studies suffer (Lachenbruch and Sass, 1980; Rice, 1992), but results. Only a few incorporate fault-zone observa- from map-view bins with sizes ≥2 km perpendicular others argue that an along-strike anomaly exists tions directly (Axen and Selverstone, 1994; Vermilye to the faults; these bins limit the ability to separate (Scholz, 2000) or that heat-flow measurements and Scholz, 1998; Chester and Chester, 2000; Healy, “near-fault” from “background” events, and some need not be a robust indicator of fault strength

2009), and relatively few have been tested using ex- studies have reached opposite conclusions with (e.g., Saffer et al., 2003). Lithostatic Pf is suggested posed fault-zone rocks. similar or identical data sets that were binned dif- along misoriented reverse faults at seismogenic Low-angle normal faults offer the opportunity ferently (Hardebeck and Michael, 2004). In order to depths (Sibson et al., 1988), in accretionary prisms to observe fault-zone rocks developed along weak capture sufficient numbers of earthquakes, narrow, (Fagereng et al., 2010), and was likely along the faults: their generally large-magnitude slip com- near-fault bins are often longer than desirable, and Zuccale LANF that contains many shallowly dip- monly has delivered directly to the surface footwall bins along the faults narrower than 2 km yield data ping extensional veins (Collettini and Holdsworth,

2004; Smith et al., 2008). Many LANFs in the west- (Johnson and Loy, 1992; Caskey et al., 1996; Axen ities. Aftershocks around the San Andreas fault ern U.S. Cordillera experienced large-volume fluid et al., 1999) and presence of frictional melt-rock may reflect a stress field rotated to higher angles to flux through the fault zones (e.g., review in Axen, (pseudotachylyte) (John, 1987; Prante et al., 2014). the fault, relative to interseismic seismicity, due to 1992; Person et al., 2007; many others), but the Evidence for creep on some LANFs indicates that shear stress reduced by the main shock (Townend

levels of paleo-Pf are generally uncertain. dynamic weakening mechanisms do not apply to and Zoback, 2001; Hardebeck and Michael, 2004).

Elevated paleo-Pf is difficult to prove in the ab- all (Collettini et al., 2009). We emphasize that “on-fault” seismicity in seismic sence of appropriately oriented extensional veins, Pseudotachylyte exists along both the Whipple stress-inversion literature typically includes the en- which are lacking in the detachments studied here. (Wang, 1997) and West Salton detachment faults tire damage zone and may be 2 km to >5 km from

In an Andersonian extensional stress field (withs 1 (Axen and Fletcher, 1998; Axen et al., 1998; Prante the fault in question and therefore includes both on-

vertical), subvertical tensile cracks should form if Pf et al., 2014), indicating that both slipped seismo- fault and off-fault fracturing as used here, and the

exceeds rock tensile strength, causing Pf to drop. genically at least in part. For this study, the ques- spatial resolution in seismic inversions is orders of Also, steeply dipping upper-plate normal faults tion is: what off-fault damage may have resulted magnitude larger than in our study. are common above LANFs and provide additional from earthquakes? Regardless, the static coefficient of friction on

Pf escape routes. Thus, in the absence of special The earthquake cycle can be broken into four LANFs cannot be ignored, whether or not dynamic stress conditions within the fault damage zone that parts: interseismic, preseismic, coseismic, and weakening occurs during earthquakes on LANFs: prevent hydrofracture (e.g., Rice, 1992; Axen, 1992; postseismic. Low-level seismicity occurs around static friction must be overcome in order to initiate Healy, 2009; Weak-Sandwich Models section), it many faults during the interseismic period between an earthquake rupture. Also, dynamically lowered

seems unlikely that supra-lithostatic Pf levels can major events and presumably reflects local failure friction must increase to arrest the propagating be maintained in most LANFs. Axen and Selver- in the regional stress field controlled by long-term rupture and must return to static values during the stone (1994; Strong-Sandwich Model section) cal- tectonic loading. Stress inversions of interseismic interseismic period. Thus, if LANFs have low static

culated elevated Pf (up to 0.7 of lithostatic pressure events around the San Andreas fault are interpreted friction and are misoriented with respect to the at 10 km depth) in mechanical models of brittle various ways by different authors and may show stress fields around them remain key questions. LANF slip on the Whipple fault, conditions that rotation from higher angles from the fault (60°–85°) would not lead to pure tensile failure. Cataclastic to moderate angles (40°–55°) closer to the fault, but textures along both the Whipple and West Salton low angles (~30°) are rare (see review by Hardebeck Stress Changes in Fault Damage detachments are consistent with constrained com- and Michael, 2004). Preseismic foreshocks are rare Zones or Cores minution (Sammis et al., 1987) and sublithostatic on all types of faults, and none are reported from

Pf (Luther et al., 2013). Elevated Pf may be consid- LANFs; but this may reflect the very sparse LANF Weak-Sandwich Models ered a viable mechanism for LANF slip, if no other earthquake catalog. Coseismic off-fault damage model explains data better, but should not be may be due to both rupture propagation and fault A variety of models propose that weak faults viewed as a universal solution or favored a priori. roughness. Earthquake rupture propagation (Di may slip because of a local stress rotation within the Toro et al., 2005; Rice et al., 2005) should create a fault damage zone (~100 m) or fault core (1–10 m), near-fault stress field and resulting structures that caused by strength changes perpendicular to the Dynamic Weakening and are similar in orientation to those caused by initial fault. One such “weak-sandwich model” (Fig. 1) Earthquake‑Related Damage fault propagation (Fig. 5 and the Fault and Earth- was proposed by Rice (1992) for the San Andreas quake Rupture Propagation section; Vermilye and fault and applied by Axen (1992) to LANF slip. The

Dynamic weakening during seismogenic slip Scholz, 1998). It is possible that seismic slip rates on fault zone has higher Pf than the surrounding host (Brodsky and Kanamori, 2001; Di Toro et al., 2004; nonplanar faults may create damage that is similar rocks, which, in turn, causes mean stress increase

Wibberley and Shimamoto, 2005; Rempel, 2006; to that predicted by static elastic models of wavy within the fault zone and s1 to rotate to a shallow Rice, 2006) can explain observations of low heat fault slip (Chester and Chester, 2000; Damage Due angle (~30°) adjacent to the fault. Increased mean flow around strike-slip faults (e.g., Brune et al., to Slip on Nonplanar Faults section ), but we are not stress in the fault zone prevents hydrofracture. 1993) that suggests they are weak (Lachenbruch aware of such models. Most large earthquakes are In this local stress field, the LANF has a primary and Sass, 1980). Dynamic weakening also may ex- followed by a period of aftershock activity, much Reidel shear orientation but secondary (antithetic) plain the inferred weakness of LANFs: seismicity of which is off-fault so should be reflected in off- Reidel shears, and a mineralized fault core might on some has been inferred from historical earth- fault damage. Aftershocks may reflect short-term be expected. quake studies (Abers, 1991, 2001; Wernicke, 1995; stress perturbations due to the mainshock (King Another weak-sandwich model relies on Cou- Abers et al., 1997; Axen, 1999), fault-scarp studies et al., 1994) or local stress perturbations at asper- lomb plasticity due to granular flow in the fault

Faulkner et al. (2006; see also Heap and Faulk- σ A σ A 1 ner, 2008) presented a weak-sandwich model with 1 F F 90 – ψ s a F near-fault stress rotation similar to that of Rice s F a (1992) in which rotation is caused by damage rather 20 45 R 1 R2 20 30 σ1D than Pf, with elastic parameters changing inward R through the damage zone due to progressive ten- 2 σ1D sile cracking. They characterized microfracture den- Core sity as a function of distance from faults, performed <1 , to ~1 10s t laboratory experiments on similar lithologies, and 0 m 100s o me of characterized the evolution of Young’s modulus and B ters τ B σ1 Poisson’s ratio as functions of microcrack density. Applying those elastic properties as if they were iso- τ R σ tropic in a model of fault-parallel layers that are in- 1 D F creasingly damaged toward the fault, they showed s σ D that around misoriented faults should rotate to σ σ s1 3D 1D σn ψ lower angles to the fault plane as it is approached. σ σ σ P 3 P 1 1D If damage must exist to cause stress rotation, but R2 h d σn σ damage evolution is controlled by near-fault stress 3D R 2 perturbations, then this model predicts that zones Fa closer to a misoriented fault should show overprint- Figure 2. Coulomb plasticity in the cataclastic fault core ing cracks with younger ones reflecting stress fields (green) (Mandl et al., 1977; Byerlee and Savage, 1992; Lockner progressively rotated to lower angles to the fault. (In and Byerlee, 1993; Marone, 1995) causes stress rotation over Figure 1. (A) Weak-sandwich model of Rice (1992) as applied their field area, in fact, the microcracks show little a distance of meters from the fault. Symbols and colors as in to low-angle normal faults (LANFs) by Axen (1992). Weakness Figure 1, except R1 shows primary Reidel shear orientation. and stress rotation inside the damage zone and fault core are change of orientation, being ~30° from the fault at

caused by elevated pore-fluid pressure (Pf) but may have other all distances and not providing evidence of any pro- causes (e.g., Faulkner et al., 2006). LANF (red) is surrounded by gressive stress rotation within the microfractured the Zuccale fault (Collettini and Holdsworth, 2004; weak material of the fault core (green) and fractured damage zone [Faulkner, 2007, written commun.].) Faulkner Smith et al., 2008) but does not seem applicable zone (gray). Fs and Fa are steeply dipping (60°) synthetic and antithetic hanging-wall normal faults, respectively. The LANF et al. (2006) treated the damaged rocks as elastically to the WDF or WSDF, except possibly in the upper- has the orientation of a primary Reidel shear; a secondary isotropic, which is almost certainly not true because most few km of the crust (see The Whipple and Reidel shear within the weak material is shown in blue as R2. (B) Mohr diagram showing stress state outside fault zone and tensile microfractures have preferred orientations West Salton Detachment Faults section). on LANF but without failure envelopes for intervening weak perpendicular to the least principal stress and will layers. LANF (and weak layers) have same frictional failure cause anisotropic elastic properties. criteria as surrounding crust but elevated pore-fluid pressure on and near the fault causes failure envelope to shift right, Healy (2008) addressed the anisotropy of the Strong-Sandwich Model preventing hydrofracture around LANF. Shear and normal trac- damaged rock and concluded that the opposite ef- tions are continuous across boundaries between layers, as at fect, s1 rotated to higher angles to a misoriented Axen and Selverstone (1994) proposed a point in Mohr diagram, which also represents the traction σD fault, is expected if elevated P is involved and model for slip on the Whipple detachment fault on the LANF. Maximum and minimum principal stresses, re- f

spectively, are σ1 and σ3 in the host rock, and σ1D and σ3D on the for reasonable damage patterns. Healy (2009) ex- in which the mineralized material of the fault core detachment. The detachment dip ψ is 20°. Ph and Pd are fluid tended this analysis to low-angle normal faults, as- (chlorite-epidote breccia zone) is stronger than both pressure in the host rocks and on the detachment, respectively. suming that weak, foliated (anisotropic) materials the fault and the surrounding heavily fractured (shale, clays, talc, and serpentinite) form the fault damage zone (Fig. 3). This model is based upon core and that pulses of elevated fluid pressure pass observations of conjugate faults in the chlorite-epi- core (approximately one to tens of meters away upward from the footwall, through the LANF and dote breccia zone, surrounding damage zone, and

from the LANF; Fig. 2), with s1 oriented ~45° to the into the hanging wall. This mechanism may apply deeper “background” levels and is consistent with

fault, allowing slip at a somewhat lower apparent to LANFs showing evidence of anisotropic fault s1 in a subvertical (Andersonian) orientation. The

friction value (Mandl and Fernandez Luque, 1970; cores and/or elevated Pf, such as the seismogenic conjugate faults measured have abnormally low Mandl et al., 1977; Byerlee and Savage, 1992; Lock- Woodlark detachment fault (Abers, 1991; Abers inter-fault (conjugate) angles, with many showing ner and Byerlee, 1993; Marone, 1995). et al., 1997; Floyd et al., 2001; Roller et al., 2001) or transtensile failure (opening plus shear offset).

Assuming s1 bisected the conjugate angle, then it A σ σ F 60 1 F F was oriented at high angles (55°–80°) from the WDF 1 s F s a a A (consistent with our results, General Paleostress 80

Results and Steeply Plunging Extensional s1 and 20 ~15km LANF slip sections). The transtensile failure and low Cs Ca conjugate angles are consistent with elevated (but σ1D R R sublithostatic) Pf in the mineralized zone, where hy- 45 1 2 10 drous alteration minerals epidote and chlorite form s t o 100s Brittle-PlasticTransition a large percentage of the rock (see Selverstone metersof C′ et al., 2012). Using a Griffith failure envelope and B Appliedhorizontal assuming a tensile strength of 10 MPa for the min- τ basalshear traction eralized zone, Axen and Selverstone (1994) showed τ that, for slip to occur on the WDF, Pf needed only to σD B C be moderately elevated (~0.7 lithostatic) at ~10 km s σD Fs σ R1,C′ depth but could be hydrostatic at shallower depth. 3D σ1(= σv) T o σn F F Ca a s σ1D Crustal-Scale Stress Rotation σ 3D σn F If s1 rotates from subvertical in the upper crust a to a moderate plunge in the midcrust, then LANFs Figure 3. Strong-sandwich model (Axen and Selverstone, at depth may slip without violating standard fault 1994). The fault core (green) and adjacent damage zone (gray) R2 mechanics or rock friction values. Yin (1989) formu- are continuously mineralized and healed and therefore support greater differential stress than fractured and faulted rocks lated an elastic model with s1 plunging 45° at the above and below, where differential stress is limited by cohe- brittle-plastic transition (Fig. 4), due to assumed sionless frictional failure. Pore-fluid pressure (Pf) is the same Figure 4. Crustal-scale stress rotation. (A) Andersonian, sub-

subhorizontal shear traction applied to the base of everywhere near the fault, and effective stresses are shown. vertical σ1 in the brittle crust (blue) gradually changes to 45° the brittle crust by subjacent plastic flow. Spencer Failure in the damage zone and fault core is on conjugate, plunge at brittle-plastic transition (dashed line), where sub- synthetic and antithetic (C and C , respectively), mixed mode horizontal shear traction controls orientation (Yin, 1989). C and Chase (1989) argued for similar crustal-scale s a σ1 ′ I-II tensile-shear fractures that form with low interfault an- planes in mylonites there have same orientation as primary

stress rotation resulting from lateral crustal thick- gles, obeying a Griffith criterion. Maximum principal stressσ 1 Reidel shears (R1) (Selverstone et al., 2012). (B) Mohr diagram, ness changes. Buck (1990) objected to the analy- bisects conjugate fault angles, is subvertical at all structural immediately after faults formed (under Coulomb criterion, not sis of Yin (1989), showing that the LANFs at depth levels, and is at high angle to low-angle normal fault. Other shown), assuming stress states at and above the brittle-plastic abbreviations as in Figure 1. transition are controlled by cohesionless friction (e.g., Brace would have lower resolved shear traction than and Kohlstedt, 1980). At the brittle-plastic transition (pink cir-

steeper conjugates, which he argued would form cle) the low-angle normal fault is at 45° to σ1. White circle rep- preferentially. However, the steep conjugate faults resents steep normal faults in upper crust. Other abbreviations as in previous figures. should have reverse slip in the deeper, stronger strength-depth profiles based upon experimental

brittle crust (R2 shears in Fig. 4) and, in extending rock mechanics (e.g., Brace and Kohlstedt, 1980). lithosphere, the LANF orientations probably are fa- Selverstone et al. (2012) provided support for vored by boundary conditions, nonelastic rheology, this class of models in their study of the paleo– are thought to be planes of maximum shear stress; and/or energetic considerations. Westaway (1999, brittle-plastic transition exposed in the Whipple the WDF is subparallel to structurally deeper mylo- 2001) proposed a similar, but very complex model Mountains, using sites and samples from the nitic C planes. C′ shears that formed during mylo- in which shear traction is greater on the LANF ori- upper mylonite zone, tens of meters below the de- nitization were followed by development of brittle entations. In crustal-scale stress rotation models, tachment. They argued that embrittlement in any primary Reidel shears in the same orientation and master faults are initially listric and curve gradually given place was rapid, permanent, and caused by by conjugate secondary Reidel shears (Fig. 4); if with depth, from steep dips (~60°) in the near sur- fluid infiltration and precipitation of epidote, with interpreted as conjugate Coulomb fractures, these

face to gentle dips at the base of the elastic layer s1 oriented ~45° from mylonitic C planes both orientations also imply s1 oriented ~45° from the (Fig. 4). Differential stress should be high at the before and after embrittlement. C planes are the C planes and the evolving WDF in the midcrustal base of the brittle crust, consistent with common dominant shear plane during mylonitic flow and brittle-plastic transition. Embrittlement occurred at

temperature of 380–420 °C, while Pf dropped from Earthquake rupture propagation should cause most likely near the fault in releasing bends where lithostatic to hydrostatic levels, also consistent with similar patterns of damage that will overprint dam- fault-normal compression is reduced, the failure re- high differential stress in the midcrust (Selverstone age due to initial fault propagation and that may gion will expand along and away from the fault for

et al., 2012) and suggesting that lithostatic Pf might dominate fractures measured in the field (e.g., higher friction, and the maximum principal stress not exist at higher crustal levels. Rice et al., 2005). Both the WDF and WSDF were in failure regions will be at a high angle to the fault arguably seismogenic (Dynamic Weakening and in either scenario, with relatively little variation of Earthquake-Related Damage section); so rupture orientation (20°–40°) within the region of failure Fault and Earthquake Rupture Propagation propagation may dominate over fault propagation (Fig. 6). In restraining bends, where failure is ex-

in the stress inversions, if those processes were pected only for high friction, s1 is subparallel to the The fault-propagation model of Vermilye and significant in forming the fractures studied here. regional applied stress and has similar magnitude. Scholz (1998; Fig. 5) is based upon detailed studies In large normal-fault earthquakes (mostly with of damage around small faults and considers the dips >30°), the hypocenter is generally near the σ µ stress field in the process zone ahead of propa- base of the crustal seismogenic zone (e.g., Jack- A 1 =0.25 gating fault tips, where damage accumulates due son, 1987; Jackson and White, 1989; Abers, 1991; 70 Contours: Regions to stress concentrations caused by slip gradients. Abers et al., 1997; Axen, 1999); therefore, rupture β = 70 of expectedfailure 65 for µ =0.25, 0.35,0.4 Background studies near those faults suggest that propagation should have an up-dip (mode II) com- 62 63 L the far-field stress was oriented ~30°–40° away from ponent. Earthquake ruptures on large, steep nor- 0. 72 35 80 them (measured in planes perpendicular to the mal-faults are typically several times longer than 85 90 faults and that contain the slip vectors), consistent their down-dip length; so damage formed during 93 88 4 81 with experimental results. For propagation of mode along-strike mode III rupture propagation may 0. 75 70 II fractures (where the fault tip-line is perpendicular dominate. LANFs should have a greater down-dip 0 to the slip line), stress concentrations and damage length (Wernicke, 1995); thus their rupture areas in the process zone are asymmetrical about the fault may be more equant. If LANF hypocenters are also z/L tip (Fig. 5). For mode III propagation (at the fault tip- near the base of the seismogenic zone, then LANF B σ µ =0.6 0. line that is parallel to slip), s1 should be oriented at earthquake ruptures also should propagate both up 1 5 45 45° to the fault plane (Vermilye and Scholz, 1998). dip and laterally, and mode III rupture may be less Contours:Regions dominant than for steep normal faults. For mode II of expected failure β = 45 39 for µ =0.6,0.8,0.99 propagation up the dip, we expect that our upper 31 25 L σ1d footwall sites should reflect stress orientations in 16 11 14 the compressive quadrants, with s1 at low angles 2 d 83 to the LANFs (c in Fig. 5). If along-strike mode III 0.8 66 61 propagation dominated, then s oriented at 45° to 54 σ 1 45 1 the LANF should be recorded. 0.99 0 30 c 20 ruptur z/L σ e 1c Anticipated Damage Due to Slip on Nonplanar Faults p footwall σ ropagati 0.5 structures 1 o n Chester and Chester (2000) show with an ana- Figure 6. Near-fault trajectories of σ1 around wavy faults Figure 5. Stress reorientation due to up-dip fault or earthquake lytical elastic model that the orientation and mag- (heavy red line). Thin red lines show orientation and relative

rupture propagation as modeled by Vermilye and Scholz (1998) nitude of s1 around a sinusoidally wavy fault can magnitude of σ1 near a sinusoidally wavy fault of wavelength and Rice et al. (2005). Applied maximum principal stress σ1 is change dramatically within one-half wavelength L for friction shown in boxes; shaded regions show zones of oriented at 30° to the fault in those models. Stress field near expected tensile or Coulomb failure for various friction values perpendicular to the fault. They showed near-fault the fault tip is perturbed by elastic response to the up-dip slip (from Chester and Chester, 2000). Fault-normal direction is z;

gradient. Red lines show σ1 orientations and relative magni- stress orientations for two scenarios (Fig. 6): a fric- stress perturbation is minor for z > 0.5L. (A) Weak-fault sce- tudes near the fault tip (from Vermilye and Scholz, 1998), with tionally strong fault (m = 0.6) at a low angle (45°) to nario with friction of 0.25 and σ1 at 70° to fault. Only ~20° range σ1d in the dilatant quadrant (d) and σ1c in the compressional s and a frictionally weak fault (m = 0.25) at a high of σ1 orientation is expected near fault in regions where failure quadrant (c), as expected for our footwall sites. If propagation 1 criterion is met. (B) Strong-fault scenario with friction of 0.7 angle (70°) to . Using Coulomb failure criteria was primarily along strike (in/out of page) then σ1 near the s1 and σ1 at 45° to the fault. About 45° range of σ1 orientation is fault should be 45° from it, both above and below. for intact granite, they concluded that failure is expected near fault.

THE WHIPPLE AND WEST SALTON and tilted rocks of the hanging wall are unconform- this includes chlorite-epidote altered rocks of the DETACHMENT FAULTS ably overlapped by essentially undeformed ca. fractured damage zone; Davis et al., 1986) that is 12–14 Ma old basalts. capped by an ultracataclasite layer (“microbreccia The Whipple detachment fault (WDF; Fig. 7A) The footwall of the WDF is broadly arched ledge”) up to 2 m thick. Together, these comprise in southern California is a broadly domed, shal- about a northwest-southeast axis, presumably due the fault core. The fault-core breccia, underlying lowly dipping normal fault that accommodated to isostatic rebound driven by lateral removal of fractured damage zone, and preexisting mylonites ~40–50 km of top-to-NE extension from early to the upper plate (e.g., Spencer, 1984). The eastern are all retrogressively altered by epidote and chlo- middle Miocene (Davis et al., 1986; Howard and footwall, where our sites are located, remains with rite. Clasts of mylonite are found in the breccia, and John, 1987; Davis, 1988; Davis and Lister, 1988). a generally gentle northeast dip and is composed clasts of breccia are found in the ultracataclasite, The footwall originated at ~16 ± 4 km depth, be- of mylonitic metamorphic and intrusive rocks indicating that the ultracataclasite is younger than low the brittle-plastic transition (Anderson, 1988). of mainly Precambrian or Cretaceous age (e.g., the breccia, which is younger than mylonites. My- Extension-related mylonitization began after Anderson and Cullers, 1990). lonites and brittle fault rocks have structures in- ca. 26 Ma and had ended in the central Whipple The WDF where we worked has a 5–15-m-thick dicative of top-to-NE transport (Davis et al., 1986). Mountains by ca. 19 Ma (Davis and Lister, 1988), breccia zone (up to 300 m have been reported, but Pseudotachylite fault and injection veins are com-

117°W 115°W A 34°22′30″N B r SAF W v e

W i ″ ′ SJFZ 4°N 0 3 o R 15 0 km ′3 d r a

5 4°

22 o l 11 4° aP C o YR O c 11 LakeHavasu USA e ic a f n ci Mexico SFFZ 2°N km Mylonite Copper 3 0 50 Front BasinReservoir Quaternary WP alluviumBr Latea Cenozoic wl

Va sedimentare yrocks Z llec y 33° N ito on Se Mtns Uppee rplate basement is EF WSDFmi Whipple c Bo Lowerplate basement 34°15′N wm an ent s Dextral Faults Nevada m W Majorfoldaxes a 010 20 30 km N Detach sh N 0’ California Arizona Myloniticfootwall º3 116°30′ W 116° W

33 Detachment fault Majorfoldaxes

Figure 7. (A) Simplified geologic maps of the Whipple detachment fault (A, modified from Davis, 1988, and Davis and Lister, 1988) and West Salton detachment fault (WSDF) (B, modified from Axen and Fletcher, 1998). Insets show map locations and stars show transect locations. Abbreviations in (B): EF—Elsinore fault; SFFZ—San Felipe fault zone; SJFZ—San Jacinto fault zone; WP—Whale Peak; YR—Yaqui Ridge.

mon (Wang, 1997; Hazelton, 2003), particularly We chose these two faults for study because The ultracataclasites of both faults contain clasts of along minidetachments: slip-surfaces below and both: (1) are well characterized structurally and in cataclasite, ultracataclasite, and pseudotachylyte, subparallel to the main detachment fault that share terms of age, slip sense, and magnitude of slip; indicating reprocessing of previously comminuted many of its characteristics (Axen and Selverstone, (2) have minimal subsequent tectonic overprinting; rocks. These “microbreccia ledges” are capped by 1994; Selverstone et al., 2012) and provide evidence (3) have quartzo-feldspathic footwalls that repre- sharp principal slip surfaces. The ultracataclasite for seismic slip on the WDF (McKenzie and Brune, sent typical continental midcrustal lithologies, re- layer below the WSDF commonly contains parallel, 1972; Sibson, 1975; Spray, 1992). ducing rheologic complexity; (4) probably did not internal slip surfaces as well. The WDF and subjacent mylonitic foliation are experience lithostatic fluid pressure in the brittle At most of our sites, the WSDF and WDF juxta- folded broadly about gently NE-plunging axes crust; and (5) probably developed weak, foliated pose sedimentary or volcanic rocks over granitoid (parallel to slip direction). Folding was synchro- clay-rich gouges only in the upper crust, above the or gneiss basement rocks (Figs. 8A and 8B), but nous with WDF slip (Davis, 1988; Davis and Lister, strong part of the seismogenic crust. both detachments also have areas where the upper 1988; Yin and Dunn, 1992): in general, mylonitic fo- The stable Peninsular Ranges footwall of the plates contain quartzo-feldspathic crystalline rocks liation is more tightly folded and limbs dip more WSDF suggests that it has had minimal structural (Figs. 8C and 8D). Sedimentary-volcanic deposits in steeply than the detachment fault itself, suggesting modification by isostatic footwall uplift and the these areas are generally not more than a few km that older mylonites record more fold-related, ex- WSDF itself is nowhere west dipping (Axen and thick, with a maximum of ~5.5 km in deeper parts tension-perpendicular shortening than the younger Fletcher, 1998; Kairouz, 2005; Steely et al., 2009) as of the WSDF upper-plate basins (Dibblee, 1954; detachment. would be expected if isostatic rebound had been Kerr and Beratan, 1991; Kidwell, 1991; Yin and The top-east West Salton detachment fault significant. Additionally, the two footwalls were ex- Dunn, 1992; Nielson and Beratan, 1995; Dorsey and (WSDF; Fig. 7B) in southern California bounds the humed from different crustal levels. The WDF foot- Roberts, 1996; Lutz et al., 2006; Dorsey et al., 2007, western edge of the Salton trough, with the Penin- wall resided at ~16 ± 4 km depth (Anderson, 1988) 2011, 2012; Janecke et al., 2011). Foliated and/or sular Ranges in its footwall, and was part of the Pa- prior to onset of extension and was processed clay-rich gouge at our sites appears to lie above cific–North America plate boundary system, which through the brittle-plastic transition and the entire the principal slip surfaces (Figs. 8B and 8D) and to is dominated by the dextral southern San Andreas brittle crust by WDF slip. The WSDF footwall was have been derived from upper-plate strata or base- fault farther east (Axen and Fletcher, 1998). The at ~5–10 km depth before extension (Shirvell et al., ment from shallow depths below the basins. Thus, footwall of the WSDF is mostly composed of Creta- 2009) and exposes fault-related structures formed we conclude that low-friction phyllosilicates were ceous tonalite and granitoids that are overprinted only in the brittle crust. Northern sites in the WSDF unimportant in weakening these LANFs except at by the Late Cretaceous Eastern Peninsular Ranges footwall contain Cretaceous reverse-sense mylo- shallow crustal levels. Mylonite Zone in northern exposures (Simpson, nites of the Eastern Peninsular Ranges Mylonite Chlorite and related sheet silicates are found 1984; George and Dokka, 1994; Steely et al., 2009). Zone (Simpson, 1984; George and Dokka, 1994; along the West Salton detachment several kilome- The WSDF had ~10–20 km of top-to-east normal Axen and Fletcher, 1998; Steely et al., 2009), but ters north of our study area (Haines and van der slip from at least 5 to ca. 1 Ma (Lutz et al., 2006; southern sites are in plutonic rocks that are unfoli- Pluijm, 2012). There, the fault juxtaposes basement Shirvell et al., 2009; Dorsey et al., 2012). The WSDF ated or display weak syn-to postplutonic foliation. lithologies, and gouges are dominated by smectite slipped seismogenically at least in part, as indicated Thus, southern WSDF footwall rocks can be treated derived mainly from chlorite. Haines and van der by pseudotachylyte along it (Frost and Shafiqullah, as mechanically isotropic. We do not consider Pluijm (2012) also found clay minerals in gouges 1989; Axen et al., 1998; Prante et al., 2014). hanging-wall sites because of complex rotational from high-angle normal faults in the base of the up- During the latest (final 200 k.y.) of slip on the and steep normal-fault histories that would unnec- per plate of the Whipple detachment, but they did WSDF, the San Jacinto, San Felipe, and Elsinore essarily complicate paleostress inversion. not report weak materials from the detachment it- dextral fault zones (Fig. 7B) were also active, re- Both detachments are well exposed locally self. In both places, it is likely that the gouges formed sulting in N-S shortening and gently E-plunging (Fig. 8). The upper ~6–50 cm (WSDF) or ~0.5–2 m at shallow depths, and almost certainly at tempera- folds of the WSDF (Lutz et al., 2006; Janecke et al., (WDF) of the footwalls are composed of ultracata ture <180 °C (Haines and van der Pluijm, 2012), or at 2011; Dorsey et al., 2012). However, some footwall clasites, which in turn are composed mainly of angu- <6 km depth (Low-Friction Models section). Thus, arches, such as Whale Peak (Fig. 7B), are likely to lar micron- to millimeter-scale fragments of footwall it seems unlikely that these gouges formed in the be original corrugations (Steely et al., 2009). In gen- rocks, with some larger clasts, underlain by breccia. deep seismogenic brittle crust (~10–12 km typical of eral, the south limbs of these folds are controlled Below the WSDF, clasts are mostly unaltered from rifts). For these reasons, we doubt that the materials and/or modified by young faults of the Elsinore, the Cretaceous or older protoliths, but the WDF along the Whipple and West Salton detachments San Felipe, and San Jacinto fault zones (Fig. 7B; cataclasites include chlorite- and epidote-rich frag- were inherently weak as the footwall was carried up Kairouz, 2005; Steely et al., 2009). ments of altered detachment-related mylonites. through the seismogenic crust.

Brecciated PALEOSTRESS INVERSION METHODS A upper-plate C Upper-plate granodiorite We collected data from 17 transects (from strata WSDF six areas) in the footwall of the WSDF and eight PSS transects along the WDF. Transects have good three-dimensional exposure, are distant from UPG FFoooottwwaallll-- postdetachment structures, are located at various WDFPSS ddeerriivveedd positions on the transport-parallel folds, and have b uullttrraaccaattaaccllaassiittee homogenous rock types. We mapped (on outcrop d photographs; Fig. 9), described, and measured >5000 shear and tensile fractures in one to nine UC outcrops from each transect (~65 total) and calcu- lated the structural depths below and perpendicular to the detachments. We observed and measured fractures in outcrops with different aspects Footwall Footwall Breccia in order to reduce bias and measured fractures in Breccia a few lateral transects to check for local variabil- FDZ ity. We also measured ~2000 striations and noted ~20cm the slip or separation magnitude and sense when B D possible. We documented crosscutting relationships and fracture-fill information when possible. Brecciated upper-plate Fracture lengths range from 5 to 400 cm (most are granodiorite ~30–100 cm long). Slip magnitude on the fractures Weakly layered ranges from 1 to 100 cm (most ~1–5 cm). Outcrops to foliated are generally 1–3 m high and 1–5 m wide. upper-plate Upper-plate gouge foliated clay- We separated the database on location, depth, rich gouge and relative age for inversion analysis in T-Tecto 3.0 (Žalohar and Vrabec, 2008; Žalohar, 2009). Data from different transects are not combined for paleo stress analyses in order to test for spatial changes in paleostress orientation. Initially, each outcrop WDF data set was inverted separately. We define three PSS structural units: (1) the fault core, which is com- Footwall- posed of ultracataclasite (or microbreccia) and derived underlying cataclasite and breccia; (2) the fractured ultracataclasite damage zone, where fracture density is higher than background levels; and (3) the background WSDF where fracture density is not correlated with dis- PSS tance from the fault. Outcrop-level data sets from like structural units were often combined for sev- Figure 8. Field photos of Whipple detachment fault (WDF) and West Salton detachment fault (WSDF). (A) WDF exposure west eral different reasons: (1) fewer than ten striated of Bowman’s Wash near top of antiform. View to ~NW. UPG—upper-plate gouge; PSS—principal slip surface; UC—ultracata fractures were found; (2) the results of paleo clasite; FDZ—fractured damage zone. Upper plate composed of well-cemented sandstone. Box labeled b shows approximate area of photo in (B); hammer (in box) is ~30 cm long. (B) Close view to ~W of area in inset box in (A). Clay(?)-bearing gouge stress analyses from different outcrops were simi- above the principal slip surface is layered and weakly foliated and apparently was derived from shallow upper-plate basin fill. lar; and/or (3) the fracture orientations in different Ultracataclasite below principal slip surface was derived from footwall. (C) View to ~E of WSDF on north limb of Whale Peak outcrops were similar. We also used crosscutting antiform (in upper, east fork of Nolina Canyon). Upper plate is composed of brecciated granodiorite with irregular lenses of olive-green clay-rich gouge. This granodiorite is nonconformably overlain by a shallow upper-plate basin and was no deeper relationships within outcrops to determine if data than ~2–3 km at the onset of detachment faulting. (D) Close view of box in (C). needed to be separated. If one fracture set clearly

LookingSE LookingS 0to0.24mbelowdetachment 6.85 to 7.75 mbelow detachment Faultcore Damage zone

LookingSE 1.4to2.5 mbelow detachment Damage zone

Figure 9. Annotated ArcGIS outcrop maps from three different depths below the West Salton detachment fault (June Wash transect on nose of Whale Peak; Fig. 7B). The detachment (green line) is shown in the upper left photo; fractures are shown in either red or yellow. Stereonets are examples of the output from T-Tecto, and show the orientation of the fractures (great circles), striations (dots with short lines), and slip sense (where known; arrows on short lines show hanging-wall motion). Open squares

show principal paleostress vectors from the inversions (large = σ1, medium = σ2, and small = σ3).

formed before or after another, then they were sep- is a reduced stress tensor that includes the ori- the best-fitting reduced stress tensor by minimiz- arated; but such sets were rare. entations of the three principal axes and a shape ing the angular misfit a( , measured within the fault

We ran ~400 paleostress inversions using ratio: (s2 – s3)/(s1 – s3). We generally trust results plane) between the predicted slip direction and the T-Tecto 3.0 (Žalohar, 2009). Data input includes from fracture sets with >25% known slip direction striation direction (assumed to reflect the actual the orientation of the fault plane and striations unless we had field data to support the slip direc- slip direction) (e.g., Angelier, 1984, 1989; Žalohar and the slip sense (when known). T-Tecto output tions generated by the program. Inversions find and Vrabec, 2008) . In this study, we used a ≤ 30°.

T-Tecto automatically separates heterogeneous six adjustable parameters (Žalohar, 2009), each of RESULTS databased on two criteria: (1) striae on fractures which was varied independently to assess impact must be within 30° of that predicted by the model on inversion stability. Inversions with fewer than Footwall Damage that best fits the largest number of data; and (2) the ten fractures typically are poorly constrained and friction must be less than the ratio of shear stress are not presented. Following Provost and Houston The West Salton detachment footwall typically to normal stress. Many input parameters in T-Tecto (2003), we randomly resampled five times 75% and contains a ~0.5–1 m thick layer of ultracataclasite are adjustable, and we tested the stability of sev- 90% each of a large data set (68 fractures) and a above 2–20 cm of breccia, above 2–20 m of fractured eral of our models by varying them. The only pa- moderate-size data set (33 fractures) and inverted damage zone (altered and highly fractured tonalite, rameter that significantly affected inversion results them using both coefficients of friction to assess rare cataclasite), in turn overlying Cretaceous

was the coefficient of static friction,m s. We modeled stability of inversions. The angular variance of s1 tonalite that locally has an older mylonite fabric.

each data set twice at ms = 0.17 and ms = 0.7. orientation for the large data set ranged from 0° The detachment cuts the mylonitic foliation where Paleostress inversion analysis is based on sev- to 12.7°, and the variance of the medium-size data it is present and typically dips ~10°–30° shallower eral assumptions. (1) The maximum resolved shear set ranged from 3.2° to 5.3°, with a combined aver- than the foliation (e.g., Steely et al., 2009); so its

stress (tmax) on a plane is subparallel to the slip age variance of 5.7° (Luther, 2012). Changing from trajectory was not strongly controlled by foliation vector measured in the fault plane (Wallace, 1951; high (0.7) to low (0.17) friction contributed ~1° to planes. Fracture density decreases exponentially Bott, 1959). That this assumption is generally valid 6° of variance, with more fractures being rejected away from the fault, with ~50–100 fractures/m ad- for our results is indicated by the generally small as inconsistent with a single stress tensor for the jacent to the fault, dropping to ~10–20 fractures/m percentage of fractures not fit on the basis ofa ≤ high friction value. On this basis, we conservatively by 5–10 m below the detachment. Fractures are 30°. (2) All fractures being inverted formed at the assign a maximum angular variance of 15° to our dominantly shear fractures with striae that com- same time in the same stress field. This assump- inversions, which is small compared to the ~20°– monly rake >50°. Filled veins (typically gypsum tion appears valid because fractures of different 30° ranges used to distinguish among different or carbonate) are rare and cut the older fractures. orientations commonly are mutually crosscutting. mechanical models (colored areas on stereonets, Fracture orientations are variable depending on lo- Also, fault-core breccia and ultracataclasite formed Figs. 10–12). cation along the fault, but damage zones generally during brittle detachment slip (ca. 12 m.y. maximum for the WDF and ca. 6 m.y. for the WSDF); there- A Whipple detachment B West Salton detachment fore, inverted fractures from the fault cores arguably formed over limited time spans and potential changes in the stress fields are probably predicted by the models considered in the Weak-Fault and Fault-Damage Models section. (3) Slip occurs on fractures of many orientations, not necessarily the optimum orientations as predicted by Coulomb failure criterion (Twiss and Unruh, 1998). This is supported by the fact that most sites yield well-dispersed fracture patterns. (4) Failure occurs in an n=54 n=78 isotropic rock body. Some sites have a preexisting Figure 10. Lower-hemisphere, equal-area stereonets showing all σ1 vectors for Tertiary, detachment-related mylonitic fabric or (A) Whipple detachment fault and (B) West Salton detachment fault. Vectors older foliation. However, results from foliated ver- were rotated about local low-angle normal fault (LANF) strike by an amount that brings the LANFs to horizontal. The Kamb contour interval is 2σ. Colored areas sus non-foliated sites within a given transect and show where σ1 would approximately plot for models discussed in text: Red is for from transect to transect generally are similar; so strong-sandwich model of Axen and Selverstone (1994; Fig. 3), for Andersonian this does not appear to be a problem. Brecciation stress in the uppermost crust, or for wavy-fault models of Chester and Chester (2000; Fig. 6). Gray is for Coulomb plasticity model (e.g., Byerlee and Savage, 1992; and cataclasis within the fault cores have effectively Fig. 2), near the brittle-plastic transition in crustal-scale rotation model of Yin (1989; destroyed any preexisting anisotropy in those sites. Fig. 4) or for along-strike fault or earthquake-rupture propagation (Vermilye and T-Tecto does not include formal error analysis Scholz, 1998; Rice et al., 2005). Blue is for weak-sandwich model (Rice, 1992; Axen, (Žalohar and Vrabec, 2008; Žalohar, 2009); thus, 1992; Fig. 1) or for up-dip fault and/or rupture-propagation models (Vermilye and Scholz, 1998; Rice et al., 2005; Fig. 5). Yellow areas show predicted regions of fold- we evaluated accuracy and reproducibility of inver- ing-related σ1 values (Luther and Axen, 2013). Number of σ1 vectors (n) listed under sions the following ways (Luther, 2012). T-Tecto has each net. Arrows show upper-plate transport direction.

are cut by two sets of mutually crosscutting shear A Whipple detachment fractures; one steep set and one shallow set that Faultcore Damage zone are subparallel to the detachment. Some transects have primary Reidel shears below the main fault. The Whipple detachment fault (WDF) footwall Figure 11. Lower-hemisphere, equal-area stereo typically consists of an ultracataclasite (~1–2 m nets of all σ1 vectors, separated by depth from thick) and a thick cataclasite zone (~5–15 m). Frac- each fault. (A) Whipple detachment fault; ture densities near the fault core are ~50–75 frac- (B) West Salton detachment fault. Plotting and contours as in Figure 10. tures/m and drop rapidly to ~10 fractures/m by ~5 m below the detachment. Most fractures cutting the WDF footwall are shear fractures. Similar to the WSDF, the fractures that cut the footwall appear to n=28 n=18 be mutually crosscutting, with many that are subparallel to the detachment and other steep fractures, commonly dominated by a steep set. A set B West Salton detachment Background of 1–5 m long, subvertical, unfilled tensile fractures Faultcore Damage zone with apertures of up to 2 cm are also common. These are probably late joints and are not considered further.

It does not appear that extremely elevated Pf played a role in slip on the WDF and WSDF. We found no evidence, such as common veins of any

orientation, for supralithostatic Pf along the WDF and WSDF. This is consistent with breccia and ultra cataclasite grain-size distributions that suggest constrained comminution, in which grains do not n=22 n=27 n=19 lift and roll over or slide past one another (Luther et al., 2013). Axen and Selverstone (1994) calcu-

lated hydrostatic to elevated (but sublithostatic) Pf on the basis of conjugate opening-shear fractures and to the models considered. Most stress fields Fault and Fault-Damage Models section. Fewer in the upper footwall of the WDF, and Selverstone have a shape ratio ≤0.3 (68% from the WSDF and fold-related stress fields were obtained and are

et al. (2012) suggested, using fluid inclusions, that 86% from the WDF), indicating that s1 was distinct discussed second: s1 vectors plunging gently or

WDF Pf fell from lithostatic to approximately hydro from s2 and s3, which were subequal. For these low moderately in girdles perpendicular to transport

static during rapid embrittlement at the brittle- shape ratios, the positions of s2 and s3 may not be direction (e.g., Luther and Axen, 2013). The few ex- plastic transition. robust; thus they are not presented here. tensional stress fields that imply the wrong sense In this section, we first describe the general in- of shear on the LANFs are discussed last.

version results in terms of orientations of footwall Fifty-two WDF footwall s1 vectors (Fig. 10A)

Paleostress Fields and Their Origins s1 vectors (Fig. 10). We then focus on extensional define a fairly conical distribution with the dens- stress fields, which are most common and yield ap- est cluster plunging nearly vertically to steeply

General Paleostress Results propriate shear sense on the detachments: s1 vec- northeast. Only a few plunge moderately to gently tors lie in or near a vertical girdle in the upper-plate northeast. Several plunge moderately to gently Paleostress inversion results are shown on transport direction and plunge in that direction. northwest or southeast and suggest a weak girdle lower-hemisphere, equal-area stereonets in Fig- Several extensional vectors plunge steeply in the perpendicular to slip direction and fold hinges.

ures 10–12 as s1 orientations, in which the s1 vec- direction opposite of upper-plate transport, but Three plunge very gently either east or west. WSDF

tors were rotated about the detachment strike such these are mostly <10° from vertical and probably footwall s1 vectors (Fig. 10B) cluster less tightly and that the detachment is horizontal, allowing easy reflect inversion error. These results are compared form a weak, asymmetric girdle perpendicular to comparison among inversions from different sites to the mechanical models reviewed in the Weak- the transport direction and fold axes. The girdle

A Whippledetachment 2001). The strong-sandwich model assumes that Anticlinecrest Syncline keel N-facing limb S-facing limb mineralization healed and strengthened the LANF surroundings; thus, it allows slip under hydrostatic

Pf in the shallowest crust and requires only moder-

ately elevated Pf at seismogenic depths (Axen and Selverstone, 1994), and the model is consistent with normal friction values and Andersonian stress orientation. Crustal-scale stress-rotation models (Fig. 4; Crustal-Scale Stress Rotation section; e.g., Yin 1989; Westaway, 1999) also are consistent with n=10 n=10 n=23 n=8 Andersonian extensional stress fields in the upper brittle crust but do not predict formation of LANFs B West Salton detachment except near the brittle-plastic transition. There- Anticlinecrest Syncline keel N-facing limb fore, a mechanism, such as a rolling hinge (e.g., Spencer, 1984; Buck, 1988; Wernicke and Axen, 1988) or crustal-scale domino-style fault rotation (e.g., Davis, 1983), is required that allows LANFs to propagate into the upper crust if they have low initial dips only in the midcrust. If such a mechanism exists, then our results, particularly combined with those of Selverstone et al. (2012), who concluded

that s1 was 45° from the WDF in the midcrust, are n=10 n=18 n=50 consistent with crustal-scale stress rotation. The curvature and depth range of stress rotation is un- Figure 12. Lower-hemisphere, equal-area stereonets of all σ1 vectors, separated by position on folds. (A) Whipple detachment fault known but may not be as smooth as simple elastic (WDF); (B) West Salton detachment fault (WSDF). Plotting and contours as in Figure 10. model results suggest. Healy (2009) argued that, if frictionally weak, and its asymmetry are largely defined by a group field combined with Coulomb fracture theory elastically anisotropic, foliated fault gouge is pres-

of <10 s1 vectors that plunge gently south relative should produce initially steep normal faults, which ent along faults, then s1 may rotate to higher angles to the WSDF. are common from map to meter scales, consis- to the fault, contrary to many fault damage-zone

tent with steep s1 (Axen and Selverstone, 1994; models that assume mechanical isotropy. Healy

Strong-Sandwich Model section). As discussed in (2009) envisioned Pf pulses moving from LANF

Steeply Plunging Extensional σ1 and LANF Slip The Whipple and West Salton Detachment Faults footwalls, into the LANF core and then into the section, weak materials are generally not present hanging wall, consistent with observations from

Most inversions from both faults yield s1 orien- along these detachments, and foliated gouge that LANFs that have weak, foliated materials in their

tations that are at high angles to the LANFs (~70°– we have seen appears to have been derived mainly core and evidence of high Pf (tensile veins). Thus 90°) and plunging toward the direction of slip. from supracrustal rocks (sedimentary and volcanic) his model may be applicable to the WDF and WSDF.

Steep s1 orientations are consistent with Anderso- and subjacent basement that were shallow at the However, we argue above that such materials were nian stress orientations in the brittle crust (Ander- onset of LANF slip. Furthermore, we have no evi not present along these faults except at shallow

son, 1942), with the strong- sandwich model (Axen dence for lithostatic Pf, and most fractures in the crustal levels, and we do not have evidence for

and Selverstone, 1994) and/or with weak materials upper footwalls are shear fractures. Under normal strongly elevated Pf (Axen and Selverstone, 1994; that cannot support significant shear traction along fault-mechanical theory that ignores cohesive and Selverstone et al., 2012; Luther et al., 2013). Thus, the LANFs. tensile strength of fault surroundings, slip at the we do not favor the Healy (2008, 2009) model for

We favor the strong-sandwich model of Axen low dips of these LANFs (or at high angles to s1) these LANFs. In addition, the migrating Pf pulses and Selverstone (1994; Fig. 3) to explain the steep would be difficult with normal friction and hydro- envisioned by Healy (2009) predict that LANF slip

s1 orientations. An Andersonian extensional stress static fluid pressure (e.g., Collettini and Sibson, events should predate slip on steep hanging-wall

faults, whereas the sparse seismic record of LANF the fault cores supports constrained comminution, areas where failure is most likely; so the model is events suggests that the opposite occurs: slip on in which the cataclasites are sufficiently compact consistent with our results but difficult to evaluate steep faults triggers LANF slip (Axen, 1999). that grains cannot ride up and roll or slide over one completely with our data. In particular, the wavi- another (Luther et al., 2013). Luther et al. (2013) in- ness (parallel to transport direction) of the LANFs in

terpreted this as indicating that Pf was too low to our study is not known, but we suspect it is small. Gently to Moderately Plunging reduce the effective normal stress to levels allow- Small-offset (<1 km) faults become less rough with

Extensional σ1 Results ing granular flow, which is consistent with a rapid increasing slip (Sagy et al., 2007), but how to ex-

drop of Pf to hydrostatic levels during rapid em trapolate such results to faults with tens of kilome-

Very few s1 vectors plunge gently in the brittlement in the brittle-plastic transition (Selver- ters of slip is unclear. At the outcrop scale, the WDF upper-plate transport direction (Fig. 10), as predicted stone et al., 2012). However, a brief period(s) of gran- and WSDF appear to be very smooth and orienta- by most weak-sandwich models (Weak-Sandwich ular flow and Coulomb plasticity cannot be ruled out tion differences over tens of meters are well below Models section and Fig. 1; Rice, 1992; Axen, 1992; entirely. measurement accuracy of a hand-held compass. Faulkner et al., 2006). Thus, this class of models is Two other explanations for fractures yielding The generally small variation of ultracataclasite

not supported by our results, especially given that moderately plunging s1 vectors at sites outside the thickness over many kilometers is consistent with

most extensional s1 vectors make high angles to fault core also imply that those fractures may be low roughness because significant asperities might the LANFs. relatively old. Along-strike fault or earthquake rup- be expected to disrupt or remove the ultracata Fault- and earthquake rupture–propagation ture propagation, in mode III (Fault and Earthquake clasite layer as slip juxtaposes asperities. The WDF

models also predict low angles between s1 and Rupture Propagation section), could cause s1 to is broadly arched about a northwest-southeast the LANFs, if mode II, up-dip propagation of either be oriented ~45° to the faults (e.g., Vermilye and axis, probably due to isostatic rebound of the foot- type was important (Vermilye and Scholz, 1998; Scholz, 1998). If due to along-strike LANF propaga- wall, but our study sites are located on the north- Rice et al., 2005; Fig. 5). These models generally tion, then the fractures inverted would be some of east side of this arch in order to avoid fractures

assume that regional s1 was oriented at a low an- the most early-formed ones. If, however, the frac- related to footwall uplift (compare to Axen et al., gle to the fault in question, raising concerns about tures formed due to earthquake rupture propaga- 1995; Selverstone et al., 1995; Wawrzyniec et al., their direct applicability to these LANFs, because tion, then they could be any age (within the period 2001). Last, if the fractures used in our inversions our most common results suggest that assumption of fault activity). Both are difficult to reconcile with formed due to transport-parallel waviness of the

is invalid. In the absence of propagation modeling the very few s1 vectors that plunge gently as ex- LANFs, then the fact that we obtain broadly similar

with regional s1 at a high angle to the fault, we ten- pected from up-dip fault or rupture propagation. stress tensors at sites separated by ~3–10 km in the tatively conclude that up-dip (mode II) fracture or Alternatively, Selverstone et al. (2012) argued direction of transport (Fig. 7) suggests that the half

rupture propagation was not important in forma- that s1 was oriented ~45° from WDF mylonitic C wavelength is greater than that. tion of the damage zones. planes during late mylonitic shearing in the crustal Chester and Chester (2000) did not present re-

A somewhat greater number of paleo-s1 vec- brittle-plastic transition, and that this orientation sults for the combination of a strong fault (high fric-

tors plunge moderately in the transport direction was maintained in the midcrust during embrittle- tion) at a high angle to regional s1; so the predicted

(Fig. 10). Coulomb plasticity predicts s1 at ~45° to ment. It is not clear that this explanation is suitable variation in stress-field orientation is unknown for the fault plane (Fig. 2; e.g., Mandl et al., 1977; Byer- for the WSDF because its footwall was at ~5–10 km the case we favor (Steeply Plunging Extensional

lee and Savage, 1992). A few of our moderately in- depth at the onset of extension, so probably at s1 and LANF slip section). However, their model’s

clined s1 vectors are from fractures cutting the fault least a few kilometers above the brittle-plastic tran- mechanical basis and results are consistent with core (Fig. 11), where granular materials are present sition. Fracture sets that formed in this setting also our conclusion of strong LANFs at a high angle

and Coulomb plasticity is permitted, but any gran- would be early, and this stress orientation during to the regional s1. The variation of that angle de- ular flow must have ended by the time those frac- embrittlement would have been consistent with creases for increasing friction and, for a fault at 70°

tures formed, and most s1 vectors from the cores Coulomb plasticity, if it occurred. to regional s1 with friction ≥0.4, is largest directly

are steep. Most moderately plunging s1 vectors are adjacent to the fault, with angular variation of 40° from fractures below the fault core (Fig. 11), in ei- along the fault over one wavelength (their fig. 4). ther the fractured damage zone or from background Slip on Nonplanar LANFs The variation would be smaller within the region of sites. These may reflect stress rotation in the dam- anticipated failure. Our inversions show compara

age zones, maintaining stress continuity with the The model of Chester and Chester (2000; Fig. 6) ble or closer clustering of s1 orientations in the fault cores, if granular flow occurred there before for damage arising from slip on a wavy fault pre- slip-parallel directions (Fig. 10). In addition, regions

fractures crosscut the cores. Grain-size analysis of dicts high angles between s1 and the LANFs in the of failure predicted in their models involve tensile

s3, a stress state that is nearly identical to that in- limb. This implies a component of upward mo- Andreas fault system (e.g., Johnson et al., 1994; ferred by Axen and Selverstone (1994; compare fig. tion of the core of the fold relative to the limbs but Savage et al., 1994), which overlapped in time with 7b of Chester and Chester, 2000 to our Fig. 3B). may simply reflect rotation of generally subvertical latest (final ~200 k.y.) WSDF slip (Lutz et al., 2006;

(Andersonian) s1 during rotation of the detachment Dorsey et al., 2012). Thrusting apparently alternated to horizontal for plotting on the stereonets. Mod- with WSDF slip and extensional stress fields due to Folding-Related Stress Fields erately to gently northeast-plunging extensional the earthquake cycle because many sites cannot be

s1 vectors were obtained only from northwest-dip- fit by a single stress tensor (Luther and Axen, 2013).

Some paleo-s1 vectors lie in girdles perpen- ping fold limbs, but the reason for this is not clear. Most s1 vectors that plunge moderately north rela- dicular to the transport direction of the WDF and Folds of the WSDF plunge gently east, paral- tive to the WSDF lie on north-facing fold limbs (Fig. WSDF. Both faults are folded about axes plunging lel to upper-plate transport, and are open. Unlike 12) and were rotated ~30° (the approximate dip of parallel to transport, and these vectors are inter- the WDF, the south-dipping limbs are modified the WSDF there) to make Figure 12; so these vectors preted as reflecting fold-related stress fields (e.g., or controlled by younger, mostly dextral or dex- may simply reflect fracturing in extensional stress Luther and Axen, 2013). Dispersion perpendicular tral-oblique faults such as the San Felipe fault (Fig. fields after folding and north tilting of the WSDF. to fold axes is seen in inversions from the core and 7B; Steely et al., 2009) and faults on the south side damage zone of both faults (Fig. 11) but is greater of Whale Peak (too small to show in Fig. 7B; e.g., for the WSDF than for the WDF. “Background” sites Kairouz, 2005). We did not use sites from south-dip- Extensional Stress Fields with from the WSDF footwall show little fold-related ping limbs due to this complication. Dextral strike Incorrect Shear Sense dispersion. slip on these young strands of the San Andreas

As discussed above, the WDF itself is less fault system overlapped in time with latest (final A few extensional stress fields 1 orientations tightly folded than extension-related mylonites ~200 k.y.) WSDF slip (Lutz et al., 2006; Janecke are located in the wrong quadrant for the known indicating a three-dimensional general strain field et al., 2011; Dorsey et al., 2012), but folding may sense of slip on the LANFs (Fig. 10), implying top- in which folding and normal slip on the WDF were have begun earlier because WSDF slip was con- to-west shear traction on the WSDF (six of 78) or synchronous. Of seven sites on the WDF (Fig. 12), current with southern San Andreas fault slip (Axen top-to-southwest shear on the WDF (seven of 52).

four are near an antiformal crest (yielding ten s1 and Fletcher, 1998), implying a regional stress field About half of these lie within 15° of the vertical

vectors), and three sites (one each) are in a syn- with s1 north-south. plane that separates appropriate shear senses (top-

formal keel (ten s1 vectors), a north-facing limb Orientations of s1 vectors were determined east or top-northeast) from incorrect ones; thus,

(23 s1 vectors) and a south-facing limb (eight s1 from five sites along the WSDF (Fig 7B), with one these orientations are admissible based upon our

vectors). Orientation of s1 is most dispersed per- site on an antiformal crest (ten s1 orientations), one error analysis (Paleostress Inversion Methods sec-

pendicular to the fold axis in the antiformal crest site in a synformal trough (18 s1 orientations), and tion; Luther 2012). The remaining few presumably

sites, with both steep s1 and steep to moderate three sites on north-dipping limbs (50 s1 orienta- record localized complexities in the footwall stress northwest plunges relative to the WDF. Only about tions) (Fig.12). There is only moderate dispersion or strain fields.

five moderately northwest-plungings 1 vectors perpendicular to WSDF fold axes of s1 from anti- from antiformal crest sites are far enough from the formal crest and synformal trough sites (Fig. 12) orientations predicted by LANF-slip models to be except one gently southeast-plunging outlier from CONCLUSIONS clearly unrelated to northeast-directed slip on the the synclinal trough sites.

WDF. Why only northwest plunges are obtained is Several WSDF s1 vectors from north-dipping Inversions for reduced stress tensors were

unclear. Orientations of s1 vectors in the synformal fold limbs plunge gently south or moderately north performed on fracture orientation data sets in the trough are mostly steep and fit generally with an (Fig. 12). Luther and Axen (2013) concluded that the upper footwalls of the Whipple and West Salton de-

extensional stress field, except twos 1 vectors that gently south-plunging s1 vectors reflect thrusting tachment faults. Most inversions resulted in exten-

plunge gently northwest or southeast. These two stress fields related to flexural slip (top-to-south) sional stress fields withs 1 oriented at a high angle may record fold-related horizontal shortening. Ori- on the detachment during folding, consistent with to the LANFs and with a steep plunge, compatible

entations of s1 on northwest- and southeast-facing many north- to north-northeast–plunging striations with Andersonian stress fields. Much smaller num- limbs range from perpendicular to the WDF to on the WSDF on the north limbs of folds (Axen and bers of inversions yielded moderately plunging

steeply northwest or southeast plunging, respec- Fletcher, 1998; Steely et al., 2009). Luther and Axen extensional s1, and very few yielded gently plung-

tively, implying small components of top-north- (2013) argue that this thrusting stress field reflects ing extensional s1 vectors. In addition, some frac- west traction on the northwest-dipping limb and north-south shortening within the regional strike- ture sets appear to record stress related to exten- top-southeast traction on the southeast-dipping slip stress-strain field related to the dextral San sion-perpendicular folding.

Extensional stress inversions were compared identical to that proposed by Axen and Selverstone Angelier, J., 1984, Tectonic analysis of fault slip data sets: Jour- nal of Geophysical Research, v. 89, p. 5835–5848, doi:10 to mechanical models for slip on weak faults and (1994) based upon field measurements and me- .1029/JB089iB07p05835. support best a strong-sandwich model of the type chanical reasoning. Angelier, J., 1989, From orientation to magnitudes in paleo proposed by Axen and Selverstone (1994) for de- The paucity of s vectors inclined at low an- stress determinations using fault slip data: Journal of 1 Structural Geology, v. 11, p. 37–50, doi:10 .1016 /0191 -8141 tachment slip in the brittle crust. In that model, gles to the LANFs is not consistent with fractures (89)90034-5. LANFs can slip with normal friction values (~0.6) forming due to up-dip (mode II) fault propagation Axen, G.J., 1992, Pore pressure, stress increase and fault weakening in low-angle normal faulting: Journal of Geophysi- under hydrostatic to only moderately elevated Pf (Vermilye and Scholz, 1998) or earthquake-rupture cal Research, v. 97, p. 8979–8991, doi:10 .1029/92JB00517 . and mineralization keeps the materials surround- propagation (Rice et al., 2005). Along-strike (mode Axen, G.J., 1993, Ramp-flat detachment faulting and low-angle ing the LANF relatively strong. III) propagation may have controlled moderately normal reactivation of the Tule Springs thrust, southern Moderately plunging s1 vectors from the WDF Nevada: Geological Society of America Bulletin, v. 105, plunging s1 vectors, but this is difficult to reconcile may reflect older fracture sets formed close to the p. 1076–1090, doi:10 .1130 /0016 -7606 (1993)105 <1076: with the sparse gently plunging results. RFDFAL>2.3.CO;2. brittle-plastic transition where elastic modeling Axen, G.J., 1999, Low-angle normal fault earthquakes and trig- (e.g., Yin, 1989) and field and lab studies (Selver- gering: Geophysical Research Letters, v. 26, p. 3693–3696, ACKNOWLEDGMENTS doi:10.1029/1999GL005405. stone et al., 2012) suggest s1 was moderately This research was supported by U.S. National Science Founda- Axen, G.J., 2004, Mechanics of low-angle normal faults, in plunging. Combined, these results support rotation tion grants EAR-0809638 (G.J.A.) and EAR-0809220 (J.S.) and Karner, G., Taylor, B., Driscoll, N., and Kohlstedt, D.L., eds., of the stress across the thickness of the brittle crust, salary from New Mexico Institute of Mining and Technology Rheology and Deformation of the Lithosphere at Conti- but the depth range and curvature of this rotation (G.J.A. and A.L.) and University of New Mexico (J.S.). Work nental Margins: New York, Columbia University Press, are not well constrained. Application of this con- on the WSDF was greatly aided by the staffs of Anza Borrego p. 46–91. Desert State Park (ABDSP), especially George Jefferson, and Axen, G.J., and Fletcher, J.M., 1998, Late Miocene–Pleistocene cept to the WSDF is uncertain because its footwall Agua Caliente County Park (sampling and camping permits extensional faulting, northern Gulf of California, Mexico originated at <10 km depth at onset of extension, and access to the ABDSP library and research lab). Reviews and Salton Trough, California: International Geology Re- above the brittle-plastic transition, but the exact by C. Scholz and C. Wibberley of an early version were very view, v. 40, p. 217–244, doi:10 .1080 /00206819809465207 . Axen, G.J., and Selverstone, J., 1994, Stress state and depth is poorly constrained. helpful, and we thank J. Spencer for a recent review. N. Khalsa and A. 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